Seismic analysis of subterranean formations is a mature technology. Elastodynamic waves generated by a seismic source at a known location are measured with arrays of sensors. Based on the measurements, it is possible to detect, for example, the presence of fractures, stresses, and sedimentation boundaries in the formation. Preferably, the seismic source is relatively powerful because it is sometimes desirable to obtain measurements at distant sensors, and elastodynamic waves at certain frequencies are rapidly attenuated in the subterranean environment. Higher frequency elastodynamic waves are generally attenuated more rapidly than lower frequency elastodynamic waves. The seismic source should preferably be adjustable because different types of elastodynamic waves and different frequencies of elastodynamic waves are useful for different types of measurements. The seismic source should preferably be consistent because variations in measurements due to inconsistent source waves complicates analysis.
Known types of seismic sources include explosive charges, perforation charges, air guns, implosive bottles, orbital vibrators, axial vibrators, pneumatic shakers, PZT shakers, tube wave reflector/converters, axial spring-loaded hammer/anvil, and EM actuated hammer/anvil. These seismic sources can be divided into two categories: (1) sources which introduce acoustic waves into the borehole fluid; and (2) sources which are clamped against the borehole wall. All of the known seismic sources have some limitations. For example, simply introducing energy into the borehole fluid has limited efficiency in exciting waves in the formation because of the relatively high acoustic impedance between the fluids and the formation. The impedance causes most of the energy to be reflected back toward the borehole. Much of the energy introduced into the borehole fluid generates tube waves that propagate only within the borehole. Less energy is trapped in the tube wave when the excitation is caused by an impact on the borehole wall. The adjustability of explosive charges is also limited because a relatively wide and uncontrollable spectrum of frequencies is generated by an explosive charge. Further, obtaining consistency between explosive charges may require great precision in manufacturing, and therefore be impractical. Explosive charges also require time consuming preparations between tests, including placement of a new charge. In other words, it is not practical to detonate explosive charges in succession because a charge is likely to damage or ignite any nearby charges that would be used for subsequent firing. It would therefore be desirable to have an improved seismic source.